Systems and methods for determining calibration values for atmospheric sensors that provide measured pressures used for estimating altitudes of mobile devices

ABSTRACT

Determining calibration values for atmospheric sensors that provide measured pressures used for estimating altitudes of mobile devices. Particular systems and methods determine if any uncalibrated reference-level pressure estimates associated with an unstable pressure sensor should not be used when calibrating the unstable pressure sensor, and calibrate the unstable pressure sensor using all of the uncalibrated reference-level pressure estimates except any uncalibrated reference-level pressure estimate that should not be used when calibrating the unstable pressure sensor.

TECHNICAL FIELD

Aspects of this disclosure generally pertain to positioning of mobiledevices.

BACKGROUND

Determining the exact location of a mobile device (e.g., a smart phoneoperated by a user) in an environment can be quite challenging,especially when the mobile device is located in an urban environment oris located within a building. Imprecise estimates of the mobile device'saltitude, for example, may have life or death consequences for the userof the mobile device since the imprecise altitude estimate can delayemergency personnel response times as they search for the user onmultiple floors of a building. In less dire situations, imprecisealtitude estimates can lead a user to the wrong area in an environment.

Different approaches exist for estimating an altitude of a mobiledevice. In a barometric-based positioning system, altitude can becomputed using a measurement of pressure from a calibrated pressuresensor of a mobile device along with ambient pressure measurement(s)from a network of calibrated reference pressure sensors and ameasurement of ambient temperature from the network or other source. Anestimate of an altitude of a mobile device (h_(mobile)) can be computedby the mobile device, a server, or another machine that receives neededinformation as follows:

$\begin{matrix}{{h_{mobile} = {h_{sensor} - {\frac{{RT}_{remote}}{gM}{\ln \left( \frac{P_{sensor}}{P_{mobile}} \right)}}}},} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$

where P_(mobile) is the estimate of pressure at the location of themobile device by a pressure sensor of the mobile device, P_(sensor) isan estimate of pressure at the location of a reference pressure sensorthat is accurate to within a tolerated amount of pressure from truepressure (e.g., less than 5 Pa), T_(remote) is an estimate oftemperature (e.g., in Kelvin) at the location of the reference pressuresensor or a different location of a remote temperature sensor,h_(sensor) is an estimated altitude of the reference pressure sensorthat is estimated to within a desired amount of altitude error (e.g.,less than 1.0 meters), g corresponds to the acceleration due to gravity,R is a gas constant, and M is molar mass of air (e.g., dry air orother). The minus sign (−) may be substituted with a plus sign (+) inalternative embodiments of Equation 1, as would be understood by one ofordinary skill in the art. The estimate of pressure at the location ofthe reference pressure sensor can be converted to an estimatedreference-level pressure that corresponds to the reference pressuresensor in that it specifies an estimate of pressure at the latitude andlongitude of the reference pressure sensor, but at a reference-levelaltitude that likely differs from the altitude of the reference pressuresensor. The reference-level pressure can be determined as follows:

$\begin{matrix}{{P_{ref} = {P_{sensor} \times {\exp \left( {- \frac{{gM}\left( {h_{ref} - h_{sensor}} \right)}{{RT}_{remote}}} \right)}}},} & \left( {{Equation}\mspace{14mu} 2} \right)\end{matrix}$

where P_(sensor) is the estimate of pressure at the location of thereference pressure sensor, P_(ref) is the reference-level pressureestimate, and h_(ref) is the reference-level altitude. The altitude ofthe mobile device h_(mobile) can be computed using Equation 1, whereh_(ref) is substituted for h_(sensor) and P_(ref) is substituted forP_(sensor). The reference-level altitude h_(ref) may be any altitude andis often set at mean sea-level (MSL). When two or more reference-levelpressure estimates are available, the reference-level pressure estimatesare combined into a single reference-level pressure estimate value(e.g., using an average, weighted average, or other suitable combinationof the reference pressures), and the single reference-level pressureestimate value is used for the reference-level pressure estimateP_(ref).

The accuracy of the estimated altitude depends on the accuracy of eachmeasured pressure, P_(sensor), from each atmospheric sensor.Unfortunately, each atmospheric sensor is unstable and susceptible todrift over time. Drift is a phenomenon whereby the unstable sensor'smeasurements of pressure deviate from the true values of pressure overtime. Low amounts of drift can be tolerated when a floor-level accuracyis required for estimated altitudes. However, drift will often reachamounts that cannot be tolerated. Thus, each unstable atmospheric sensormust be calibrated to account for drift on a regular basis.

Some approaches for calibrating an unstable atmospheric sensor comparereference-level pressure estimates of the unstable atmospheric sensorwith reference-level pressure estimates of a stable atmospheric sensorover time, and determine how to calibrate the unstable atmosphericsensor based on the comparison. For example, one approach forcalibrating an unstable atmospheric sensor assumes that differencesbetween reference-level pressure estimates of the unstable atmosphericsensor and reference-level pressure estimates of a stable atmosphericsensor over time are caused by drift, and those differences can be usedas a calibration value that is applied to future reference-levelpressure estimates of the unstable atmospheric sensor to account fordrift. Occasionally, drift is not the only cause for the differencebetween a reference-level pressure estimate of the unstable atmosphericsensor and a reference-level pressure estimate of the stable atmosphericsensor. For example, occasional localized pressure difference anomaliesfrom instrument failure of the unstable atmospheric sensor, severeweather patterns in an area around the unstable atmospheric sensor thatcreate large pressure gradients between the reference sensor and theunstable sensor, the time of day (e.g., midday heating in the areaaround the atmospheric sensor), or other localized circumstances canproduce reference-level pressure estimates that should not be used whencalibrating the unstable atmospheric sensor. Systems and methods areneeded to identify when localized anomalies are influencingreference-level pressure estimates of an unstable atmospheric sensor sothose reference-level pressure estimates can be excluded fromconsideration when the unstable atmospheric sensor is calibrated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an operational environment in which atmospheric sensorsthat provide measured pressures used for estimating altitudes of mobiledevices are calibrated.

FIG. 2A and FIG. 2B illustrate consequences of using pressuremeasurements of an unstable atmospheric sensor that are affected bylocalized circumstances when calibrating the unstable atmosphericsensor.

FIG. 3A and FIG. 3B illustrate benefits of detecting pressuremeasurements of an unstable atmospheric sensor that are affected bylocalized circumstances when calibrating the unstable atmosphericsensor.

FIG. 4 depicts a process for determining calibration values foratmospheric sensors that provide measured pressures used for estimatingaltitudes of mobile devices.

FIG. 5A and FIG. 5B depict different processes for determining if theuncalibrated reference-level pressure estimates include any uncalibratedreference-level pressure estimate that should not be used whencalibrating the unstable pressure sensor.

FIG. 6A and FIG. 6B depict different processes for identifying at leastone uncalibrated reference-level pressure estimate that should not beused when calibrating the unstable pressure sensor.

FIG. 7A and FIG. 7B illustrate different processes for identifying atleast one uncalibrated reference-level pressure estimate that should notbe used when calibrating the unstable pressure sensor.

FIG. 8 illustrates components of a transmitter, a mobile device, and aserver.

DETAILED DESCRIPTION

Approaches for determining calibration values for atmospheric sensorsthat provide measured pressures used for estimating altitudes of mobiledevices are described herein. Attention is initially drawn to FIG. 1,which depicts an environment 100 in which an altitude of a mobile device120 is estimated using a measured pressure at the position of the mobiledevice 120 and measured pressures from unstable atmospheric sensors 130(e.g., weather stations) at different known altitudes in a network ofsuch sensors.

One common approach for estimating the altitude of the mobile device 120uses Equation 1 (and optionally Equation 2) described in the Backgroundsection of this disclosure. By way of example, each unstable atmosphericsensor 130 transmits pressure data (e.g., reference-level pressurescomputed by the atmospheric sensor, or measured pressures ifreference-level pressures are computed at the mobile device 120, aserver, or elsewhere). The pressure data is received by the mobiledevice 120 and/or a server for use in determining an estimated altitudeof the mobile device 120. Transmission from a particular unstableatmospheric sensor 130 may be made by any known means, including (i)transmission via a transmitter (not shown) that includes or isco-located with the atmospheric sensor 130, or (ii) any suitabletechnique for transmitting data from the atmospheric sensor 130 toanother thing (e.g., a mobile device or a server).

The accuracy of the estimated altitude depends on the accuracy of themeasured pressures from the unstable atmospheric sensors 130, which canbe affected by measurement errors due to drift of the unstableatmospheric sensors 130 over time. Thus, each of the unstableatmospheric sensors 130 must be calibrated to account for drift of thatunstable atmospheric sensor 130. One approach used to calibrate anunstable atmospheric sensor 130 involves (i) converting measurements ofpressure of the unstable atmospheric sensor 130 to a reference-levelpressure estimate for a reference-level altitude (e.g., using Equation 2described previously), (ii) converting measurements of pressure of astable (i.e., “golden”) atmospheric sensor 140 to a reference-levelpressure estimate for the reference-level altitude (e.g., using Equation2 described previously), (iii) determining a time-averaged difference ofthe reference-level pressure estimates of the unstable atmosphericsensor 130 and the reference-level pressure estimates of the stableatmospheric sensor 140, and (iv) using the time-averaged difference as acalibration value to adjust future measurements of pressure from theunstable atmospheric sensor 130 or a corresponding reference-levelpressure estimates determined from the measurements of pressure.

Unfortunately, not all measurements of pressure from the unstableatmospheric sensor 130 can be used to determine a calibration value forthe unstable atmospheric sensor 130. Occasionally, technical issues withthe unstable atmospheric sensor 130 other than drift, severe weatherpatterns in an area around the unstable atmospheric sensor 130, the timeof day (e.g., midday heating in the area around the unstable atmosphericsensor 130), or other localized circumstances can produce measurementsof pressure that should not be used when calibrating the unstableatmospheric sensor 130, since such circumstances do not affect themeasurements of pressure from the stable atmospheric sensor 140, or suchphenomena are transient and do not affect the long-term averagemeasurement. By way of example, FIG. 2A and FIG. 2B illustrateconsequences of using pressure measurements that are affected bylocalized circumstances when calibrating the unstable atmospheric sensor130. In contrast, FIG. 3A and FIG. 3B illustrate benefits of detectingpressure measurements that are affected by localized circumstances, andexcluding them from consideration when calibrating the unstableatmospheric sensor 130. As shown in FIG. 2A, the reference-levelpressure estimates of the unstable atmospheric sensor 130 includechanges in pressure over time that do not align with the reference-levelpressure estimates of the stable (“Golden”) atmospheric sensor 140. Asshown in FIG. 2B, a predefined percentage such as 50% or more of thedifferences between accurate reference-level pressure estimates for thestable atmospheric sensor 140 and calibrated reference-level pressureestimates for the unstable atmospheric sensor 130 (i.e., reference-levelpressure estimates adjusted by a calibration value determined using thepressure values shown in FIG. 2A) are nearly 6 Pa, which does not meet atypical threshold difference such as 2 Pa or less. As shown in FIG. 3A,the reference-level pressure estimates of the unstable atmosphericsensor 130 exclude the changes in pressure over time that do not alignwith the reference-level pressure estimates of the stable atmosphericsensor 140. As shown in FIG. 3B, the predefined percentage such as 50%or more of the differences between accurate reference-level pressureestimates for the stable atmospheric sensor 140 and calibratedreference-level pressure estimates for the unstable atmospheric sensor130 (i.e., reference-level pressure estimates adjusted by a calibrationvalue determined using the pressure values shown in FIG. 3A) are lessthan 2 Pa, which meets the typical threshold difference such as 2 Pa orless.

One of ordinary skill in the art will appreciate that systems andmethods are needed to identify unusable pressure values (e.g.,measurements of pressure or corresponding reference-level pressureestimates) of an unstable atmospheric sensor 130 so those pressurevalues can be excluded from consideration when the unstable atmosphericsensor 130 is calibrated. In one system and method, a stable atmosphericreference sensor is used to measure pressure and temperature over a timeperiod T, and the measured pressures are converted to reference-level(e.g., sea-level) pressures using the measured pressure, the measuredtemperature, the known altitude of the stable atmospheric referencesensor, and Equation 2 described earlier in this disclosure. Similarly,an unstable atmospheric reference sensor is used to measure pressure andtemperature over the time period T, and the measured pressures areconverted to reference-level pressure estimates using the measuredpressure, the measured temperature, the known altitude of the unstableatmospheric reference sensor, and Equation 2. In one embodiment,unusable reference-level pressure estimates (or the pressuremeasurements used to determine those reference-level pressure estimates)that correspond to the unstable atmospheric sensor can be manuallyidentified and excluded (e.g., from a technician field report or aweather report) before remaining reference-level pressure estimates areused to determine a calibration value that can be used to “calibrate”the unstable atmospheric sensor (e.g., used to adjust pressure valuesfor the unstable atmospheric sensor). In another embodiment, unusablereference-level pressure estimates (or the pressure measurements used todetermine those reference-level pressure estimates) that correspond tothe unstable atmospheric sensor can be automatically identified andexcluded before remaining reference-level pressure estimates are used todetermine a calibration value that can be used to “calibrate” theunstable atmospheric sensor. During one embodiment of an automaticapproach, a cumulative distribution function (CDF) of the time-aligneddifference between corresponding pressure values (e.g., reference-levelpressure estimates) of the stable atmospheric sensor and the unstableatmospheric sensor is determined (e.g., where the distributionrepresents the absolute value of the difference). A value of the CDF fora predefined percentage (e.g., 50%) is determined, and that value iscompared to a threshold pressure value (e.g., 2 Pa). If the value isless than the threshold pressure value, a determination is made that allof the pressure values (e.g., the reference-level pressure estimates) ofthe unstable atmospheric sensor can be considered when the unstableatmospheric sensor is calibrated (e.g., when determining a calibrationvalue). If the value is not less than the threshold pressure value, adetermination is made that some of the pressure values of the unstableatmospheric sensor cannot be considered when the unstable atmosphericsensor is calibrated, and an outlier rejection process is started toidentify unusable pressure values. Different outlier rejection processescan be used, including measuring the difference between each pressurevalue against the median of neighboring pressure values (e.g., npressure value(s) before and n pressure value(s) after the pressurevalue), and designating the pressure value as unusable if the differenceexceeds a threshold amount of pressure. The advantage of such a processis that it can detect features with sharp, sudden peaks. An alternativeprocess that uses median/mean subtracted difference data can helpidentify features that may not necessarily have sharp changes andinstead have more gradual changes. After unusable pressure values aredetermined, the remaining usable pressure values are used to compute acalibration value for the unstable atmospheric sensor.

FIG. 4 depicts a process for determining calibration values foratmospheric sensors that provide measured pressures used for estimatingaltitudes of mobile devices.

Initially, for each period of time from different periods of time, anuncalibrated reference-level pressure estimate of an unstable pressuresensor and a calibrated reference-level pressure estimate associatedwith a stable pressure sensor are determined (step 410). Theuncalibrated reference-level pressure estimate of the unstable pressuresensor is based on an uncalibrated pressure measurement that wasmeasured by an unstable pressure sensor during the period of time, andthe calibrated reference-level pressure estimate of the stable pressuresensor is based on a calibrated pressure measurement that was measuredby the stable pressure sensor during the period of time. One example ofdetermining a reference-level pressure estimate for a reference altitudeincludes receiving the reference-level pressure estimate from thepressure sensor at a server, where the reference-level pressure estimatewas generated at the pressure sensor using previously-described Equation2. By way of example, an “uncalibrated” pressure measurement may includea measurement of pressure from a pressure sensor that has not beenadjusted to account for drift. The period of time can be any amount oftime that is long enough for the unstable pressure sensor and the stablepressure sensor to generate their corresponding pressure measurementsunder similar environmental pressure conditions, which may or may notoccur at exactly the same time. Ideally, the pressure measurements wouldbe generated at the same time, but the period of time could depend onthe time of day or current weather pattern in an environment such thatthe period of time can range from a few mins (e.g., for diurnal pressurevariation of a hot afternoon), to tens of minutes (e.g., at night timewhen temperature does not have much influence in pressure patterns). Iftimes do not align, then thresholds are adjustable to handle theincreased uncertainty in the pressure measurement.

As the process advances, a determination is made as to whether theuncalibrated reference-level pressure estimates include any uncalibratedreference-level pressure estimate that should not be used whencalibrating the unstable pressure sensor (step 420). In one embodiment,an uncalibrated reference-level pressure estimate that is not to be usedis based on an uncalibrated pressure measurement affected by a localizedanomaly that is not attributable to drift of the unstable pressuresensor. Examples of localized anomalies include anomalies attributableto technical issues with the unstable pressure sensor other than drift,severe weather patterns in an area around the unstable pressure sensorthat are not in an area around the stable sensor, the time of day (e.g.,midday heating in the area around the unstable pressure sensor that isnot the same heating in an area around the stable sensor), or otherlocalized circumstances can produce measurements of pressure that shouldnot be used when calibrating the unstable pressure sensor. Examples ofprocesses for determining if the uncalibrated reference-level pressureestimates include any uncalibrated reference-level pressure estimatethat should not be used when calibrating the unstable pressure sensorduring step 420 are provided in FIG. 5A and FIG. 5B, which are describedlater.

If, during step 420, a determination is made that the uncalibratedreference-level pressure estimates do not include any uncalibratedreference-level pressure estimate that should not be used whencalibrating the unstable pressure sensor, a first calibration value forthe unstable pressure sensor is determined using all of the uncalibratedreference-level pressure estimates (step 430). In one embodiment of step430, determining the first calibration value includes the steps of: (a)for each of the periods of time, determine a difference between (i) theuncalibrated reference-level pressure estimate for the period of timeand (ii) the calibrated reference-level pressure estimate for the periodof time; and (b) compute the initial calibration value as an average,median or other combination of the differences (or a selected number ofthe differences).

If, during step 420, a determination is made that the uncalibratedreference-level pressure estimates include an uncalibratedreference-level pressure estimate that should not be used whencalibrating the unstable pressure sensor, at least one uncalibratedreference-level pressure estimate that should not be used whencalibrating the unstable pressure sensor is identified (step 440). Inone embodiment, an outlier rejection algorithm is used to identify eachof the uncalibrated reference-level pressure estimate(s) that are not tobe used when calibrating the unstable pressure sensor. Examples ofprocesses for identifying at least one uncalibrated reference-levelpressure estimate that should not be used when calibrating the unstablepressure sensor during step 440 are provided in FIG. 6A and FIG. 6B,which are described later.

If at least one uncalibrated reference-level pressure estimate thatshould not be used when calibrating the unstable pressure sensor isidentified during step 430, a second calibration value for the unstablepressure sensor is determined using all of the uncalibratedreference-level pressure estimates except any identified uncalibratedreference-level pressure estimate that should not be used whencalibrating the unstable pressure sensor (step 450). In one embodimentof step 450, determining the second calibration value includes the stepsof: (a) for each of the uncalibrated reference-level pressure estimatesexcept any identified uncalibrated reference-level pressure estimatethat should not be used when calibrating the unstable pressure sensor,determine a difference between (i) that uncalibrated reference-levelpressure estimate and (ii) the calibrated reference-level pressureestimate for the same period of time; and (b) compute the initialcalibration value as an average, median or other combination of thedifferences (or a selected number of the differences).

Determining if the Uncalibrated Reference-Level Pressure EstimatesInclude Any Uncalibrated Reference-Level Pressure Estimate that ShouldNot be Used When Calibrating the Unstable Pressure Sensor (Step 420)

FIG. 5A and FIG. 5B depict different processes for determining if theuncalibrated reference-level pressure estimates include any uncalibratedreference-level pressure estimate that should not be used whencalibrating the unstable pressure sensor.

A first example of a process for determining if the uncalibratedreference-level pressure estimates include any uncalibratedreference-level pressure estimate that should not be used whencalibrating the unstable pressure sensor during step 420 is provided inFIG. 5A, which includes the steps of: determining an initial calibrationvalue using the uncalibrated reference-level pressure estimates and thecalibrated reference-level pressure estimate (step 521 a); for each ofthe periods of time, determining an adjusted reference-level pressureestimate for the period of time by adjusting the uncalibratedreference-level pressure estimate for the period of time by the initialcalibration value (step 522 a); for each of the periods of time,determining a difference between (i) the adjusted reference-levelpressure estimate for the period of time and (ii) the calibratedreference-level pressure estimate for the period of time (step 523 a);determining if a threshold amount of the determined differences are lessthan a threshold pressure difference (step 524 a); if the thresholdamount of the determined differences are less than the thresholdpressure difference, determining that the uncalibrated reference-levelpressure estimates do not include any uncalibrated reference-levelpressure estimate that should not be used when calibrating the unstablepressure sensor (step 525 a); and if the threshold amount of thedetermined differences are not less than the threshold pressuredifference, determining that the uncalibrated reference-level pressureestimates include an uncalibrated reference-level pressure estimate thatshould not be used when calibrating the unstable pressure sensor (step526 a).

By way of example, one embodiment of determining an initial calibrationvalue during step 521 a includes the steps of: (a) for each of theperiods of time, determine a difference between (i) the uncalibratedreference-level pressure estimate for the period of time and (ii) thecalibrated reference-level pressure estimate for the period of time; and(b) computing the initial calibration value as an average, median orother combination of the differences (or a selected number of thedifferences).

By way of example, the threshold amount of step 524 a may be 50% or 80%,and the threshold pressure difference of step 524 a may be 2 Pa or 3 Pa.In one embodiment of step 524 a, the step determines if a thresholdamount of absolute values of the determined differences are less thanthe threshold pressure difference.

A second example of a process for determining if the uncalibratedreference-level pressure estimates include any uncalibratedreference-level pressure estimate that should not be used whencalibrating the unstable pressure sensor during step 420 is provided inFIG. 5B, which includes the steps of: for each of the periods of time,determining a difference between (i) the uncalibrated reference-levelpressure estimate for the period of time and (ii) the calibratedreference-level pressure estimate for the period of time (step 521 b);determining a mean or median of the differences (step 522 b);determining if a threshold amount of the determined differences arewithin a threshold pressure difference from the mean or median of thedifferences (step 523 b); if the threshold amount of the determineddifferences are within the threshold pressure difference from the meanor median of the differences, determining that the uncalibratedreference-level pressure estimates do not include any uncalibratedreference-level pressure estimate that should not be used whencalibrating the unstable pressure sensor (step 524 b); and if thethreshold amount of the determined differences are not within thethreshold pressure difference from the mean or median of thedifferences, determining that the uncalibrated reference-level pressureestimates include an uncalibrated reference-level pressure estimate thatshould not be used when calibrating the unstable pressure sensor (step525 b).

By way of example, the threshold amount of 523 b may be 50% or 80%, andthe threshold pressure difference of 523 b may be 2 Pa or 3 Pa. In oneembodiment of step 523 b, the step determines if a threshold amount ofabsolute values of the determined differences are within the thresholdpressure difference from the mean or median.

Identifying at Least One Uncalibrated Reference-Level Pressure Estimatethat Should Not be Used When Calibrating the Unstable Pressure Sensor(Step 440)

FIG. 6A and FIG. 6B depict different processes for identifying at leastone uncalibrated reference-level pressure estimate that should not beused when calibrating the unstable pressure sensor.

A first example of a process for identifying at least one uncalibratedreference-level pressure estimate that should not be used whencalibrating the unstable pressure sensor during step 440 is provided inFIG. 6A, which includes the following steps: for each of the periods oftime, determining a difference between (a) the uncalibratedreference-level pressure estimate of the time period and (b) thecalibrated reference-level pressure estimate for the period of time(step 641 a); for each of the periods of time, determining if thedetermined difference for the period of time meets a threshold condition(step 642 a); if the determined difference for the period of time meetsthe threshold condition, identifying the uncalibrated reference-levelpressure estimate as one of the uncalibrated reference-level pressureestimates that should not be used when calibrating the unstable pressuresensor (step 643 a); and if the determined difference for the period oftime does not meet the threshold condition, not identifying theuncalibrated reference-level pressure estimate as one of theuncalibrated reference-level pressure estimates that should not be usedwhen calibrating the unstable pressure sensor (step 644 a). In oneembodiment of step 642 a, the threshold condition specifies that thedetermined difference is at least a predefined amount of pressure (e.g.,2 Pa, 3 Pa, 5 Pa, or another amount) more than another difference for aprevious or subsequent time period. In another embodiment of step 642 a,the threshold condition specifies that the determined difference is atleast a predefined amount of pressure (e.g., 2 Pa, 3 Pa, 5 Pa, oranother amount) more than a median of other differences for time periodsbefore and/or after the period of time.

A second example of a process for identifying at least one uncalibratedreference-level pressure estimate that should not be used whencalibrating the unstable pressure sensor during step 440 is provided inFIG. 6B, which includes the following steps: determining an initialcalibration value using the uncalibrated reference-level pressureestimates and the calibrated reference-level pressure estimates (step641 b); for each of the periods of time, determining an adjustedreference-level pressure estimate for the period of time by adjustingthe uncalibrated reference-level pressure estimate for the period oftime by the initial calibration value (step 642 b); for each of theperiods of time, determining a difference between (a) the adjustedreference-level pressure estimate of the time period and (b) thecalibrated reference-level pressure estimate for the period of time(step 643 b); for each of the periods of time, determining if thedetermined difference for the period of time meets a threshold condition(step 644 b); if the determined difference for the period of time meetsthe threshold condition, identifying the uncalibrated reference-levelpressure estimate as one of the uncalibrated reference-level pressureestimates that should not be used when calibrating the unstable pressuresensor (step 645 b); and if the determined difference for the period oftime does not meet the threshold condition, not identifying theuncalibrated reference-level pressure estimate as one of theuncalibrated reference-level pressure estimates that should not be usedwhen calibrating the unstable pressure sensor (step 646 b). In oneembodiment of step 644 b, the threshold condition specifies that thedetermined difference is at least a predefined amount of pressure (e.g.,2 Pa, 3 Pa, 5 Pa, or another amount) more than another difference for aprevious or subsequent time period. In another embodiment of step 644 b,the threshold condition specifies that the determined difference is atleast a predefined amount of pressure (e.g., 2 Pa, 3 Pa, 5 Pa, oranother amount) more than a median of other differences for time periodsbefore and/or after the period of time.

By way of example, the processes disclosed above may be performed by oneor more machines that include: processor(s) or other computing device(s)(e.g., a personal computer or a server) for performing (e.g., thatperform, or are configured, adapted or operable to perform) each step; apressure sensor in a network of pressure sensors for measuring pressureused to determine a reference-level pressure estimate; and datasource(s) at which any data identified in the processes is stored forlater access during the processes. If there is incidental information,such as a weather report or a technician report, those reports may bestored somewhere on a storage device that can be accessed by theprocessor or computing device.

By way of example, FIG. 7A and FIG. 7B respectively illustrate outcomesof the different processes of FIG. 6A and FIG. 6B for identifying atleast one uncalibrated reference-level pressure estimate that should notbe used when calibrating the unstable pressure sensor, where theexcluded pressures are in the hashed portion of the bottom plot, whichrepresent pressures not to use in a calibration.

Technical Benefits

Mobile devices are routinely used to estimate altitudes of users basedon (i) reference pressures from a network of reference pressure sensorsand (ii) measurements of pressure from pressure sensors of the mobiledevices. Unfortunately. limitations in the functionality of the networkof reference pressure sensors and the pressure sensor of the mobiledevice can impact the accuracy of estimated altitudes, which impairsdifferent uses of the estimated altitudes (e.g., emergency response,navigation, calibration, etc.). A common technical problem is driftingof a network's reference pressure sensors and a mobile device's pressuresensors, which requires calibration over lime. Processes describedherein improve the field of calibration by determining when particularpressure estimates of a pressure sensor should not be used to calibratethe pressure sensor, which increases the accuracy of the calibrationcompared to other approaches that do not make the same determination.New and useful data is generated, such as data representing whenlocalized anomalies are influencing pressure estimates of an unstablepressure sensor so those pressure estimates can be excluded fromconsideration when the unstable pressure sensor is calibrated.Functioning of particular devices is therefore improved—e.g.,functioning of a pressure sensor that is susceptible to producingpreviously unusable data, such as pressure data withlower-than-desirable accuracy due to sensor drift, is improved byproviding for improved calibration of pressure sensors. For a givenenvironment, a more-reliable pressure sensor network can be maintainedthat is calibrated based on atmospheric characteristics (e.g., localizedanomalies) of that environment that affect calibration results comparedto prior approaches that do not consider the atmosphericcharacteristics. A more-reliable pressure sensor of a mobile device canalso be maintained. By improving calibration of pressure sensors, thefield of location determination is improved, since calibrated pressuresensors are necessary for accurate estimation of a mobile device'saltitude. More accurate estimation of a mobile device's altitude enablesquicker emergency response times or improved uses of estimatedaltitudes. The processes described herein can also be applied to otheratmospheric sensors that drift, including temperature sensors.

Other Aspects

Any method (also referred to as a “process” or an “approach”) describedor otherwise enabled by disclosure herein may be implemented by hardwarecomponents (e.g., machines), software modules (e.g., stored inmachine-readable media), or a combination thereof. By way of example,machines may include one or more computing device(s), processor(s),controller(s), integrated circuit(s), chip(s), system(s) on a chip,server(s), programmable logic device(s), field programmable gatearray(s), electronic device(s), special purpose circuitry, and/or othersuitable device(s) described herein or otherwise known in the art. Oneor more non-transitory machine-readable media embodying programinstructions that, when executed by one or more machines, cause the oneor more machines to perform or implement operations comprising the stepsof any of the methods described herein are contemplated herein. As usedherein, machine-readable media includes all forms of machine-readablemedia (e.g. one or more non-volatile or volatile storage media,removable or non-removable media, integrated circuit media, magneticstorage media, optical storage media, or any other storage media,including RAM, ROM, and EEPROM) that may be patented under the laws ofthe jurisdiction in which this application is filed, but does notinclude machine-readable media that cannot be patented under the laws ofthe jurisdiction in which this application is filed. Systems thatinclude one or more machines and one or more non-transitorymachine-readable media are also contemplated herein. One or moremachines that perform or implement, or are configured, operable oradapted to perform or implement operations comprising the steps of anymethods described herein are also contemplated herein. Method stepsdescribed herein may be order independent and can be performed inparallel or in an order different from that described if possible to doso. Different method steps described herein can be combined to form anynumber of methods, as would be understood by one of ordinary skill inthe art. Any method step or feature disclosed herein may be omitted froma claim for any reason. Certain well-known structures and devices arenot shown in figures to avoid obscuring the concepts of the presentdisclosure. When two things are “coupled to” each other, those twothings may be directly connected together, or separated by one or moreintervening things. Where no lines or intervening things connect twoparticular things, coupling of those things is contemplated in at leastone embodiment unless otherwise stated. Where an output of one thing andan input of another thing are coupled to each other, information sentfrom the output is received in its outputted form or a modified versionthereof by the input even if the information passes through one or moreintermediate things. Any known communication pathways and protocols maybe used to transmit information (e.g., data, commands, signals, bits,symbols, chips, and the like) disclosed herein unless otherwise stated.The words comprise, comprising, include, including and the like are tobe construed in an inclusive sense (i.e., not limited to) as opposed toan exclusive sense (i.e., consisting only of). Words using the singularor plural number also include the plural or singular number,respectively, unless otherwise stated. The word “or” and the word “and”as used in the Detailed Description cover any of the items and all ofthe items in a list unless otherwise stated. The words some, any and atleast one refer to one or more. The terms may or can are used herein toindicate an example, not a requirement—e.g., a thing that may or canperform an operation, or may or can have a characteristic, need notperform that operation or have that characteristic in each embodiment,but that thing performs that operation or has that characteristic in atleast one embodiment. Unless an alternative approach is described,access to data from a source of data may be achieved using knowntechniques (e.g., requesting component requests the data from the sourcevia a query or other known approach, the source searches for and locatesthe data, and the source collects and transmits the data to therequesting component, or other known techniques).

FIG. 8 illustrates components of a transmitter, a mobile device, and aserver. Examples of communication pathways are shown by arrows betweencomponents.

By way of example in FIG. 8, each of the transmitters may include: amobile device interface 11 for exchanging information with a mobiledevice (e.g., an antenna and RF front end components known in the art orotherwise disclosed herein); one or more processor(s) 12; memory/datasource 13 for providing storage and retrieval of information and/orprogram instructions; atmospheric sensor(s) 14 for measuringenvironmental conditions (e.g., pressure, temperature, other) at or nearthe transmitter; a server interface 15 for exchanging information with aserver (e.g., an antenna, a network interface, or other); and any othercomponents known to one of ordinary skill in the art. The memory/datasource 13 may include memory storing software modules with executableinstructions, and the processor(s) 12 may perform different actions byexecuting the instructions from the modules, including: (i) performanceof part or all of the methods as described herein or otherwiseunderstood by one of skill in the art as being performable at thetransmitter; (ii) generation of positioning signals for transmissionusing a selected time, frequency, code, and/or phase; (iii) processingof signaling received from the mobile device or other source; or (iv)other processing as required by operations described in this disclosure.Signals generated and transmitted by a transmitter may carry differentinformation that, once determined by a mobile device or a server, mayidentify the following: the transmitter; the transmitter's position;environmental conditions at or near the transmitter; and/or otherinformation known in the art. The atmospheric sensor(s) 14 may beintegral with the transmitter, or separate from the transmitter andeither co-located with the transmitter or located in the vicinity of thetransmitter (e.g., within a threshold amount of distance).

In some embodiments, the atmospheric sensors 14 include an unstableatmospheric sensor. In other embodiments, a stand-alone unstableatmospheric sensor (e.g., a weather station) is substituted for thetransmitter, and the stand-alone unstable atmospheric sensor includes:atmospheric sensors (e.g., a pressure sensor for measuring pressures, atemperature sensor for measuring temperatures); memory (e.g., storinginstructions for computing reference-level pressures based on themeasured pressures); processor(s) for executing instructions stored inthe memory; and any suitable interface for communicating pressure datato other things (e.g., the mobile device and/or the server).

By way of example FIG. 8, the mobile device may include: a transmitterinterface 21 for exchanging information with a transmitter (e.g., anantenna and RF front end components known in the art or otherwisedisclosed herein); one or more processor(s) 22; memory/data source 23for providing storage and retrieval of information and/or programinstructions; atmospheric sensor(s) 24 for measuring environmentalconditions (e.g., pressure, temperature, other) at the mobile device;other sensor(s) 25 for measuring other conditions (e.g., inertialsensors for measuring movement and orientation); a user interface 26(e.g., display, keyboard, microphone, speaker, other) for permitting auser to provide inputs and receive outputs; another interface 27 forexchanging information with the server or other devices external to themobile device (e.g., an antenna, a network interface, or other); and anyother components known to one of ordinary skill in the art. A GNSSinterface and processing unit (not shown) are contemplated, which may beintegrated with other components (e.g., the interface 21 and theprocessors 22) or a standalone antenna, RF front end, and processorsdedicated to receiving and processing GNSS signaling. The memory/datasource 23 may include memory storing software modules with executableinstructions, and the processor(s) 22 may perform different actions byexecuting the instructions from the modules, including: (i) performanceof part or all of the methods as described herein or otherwiseunderstood by one of ordinary skill in the art as being performable atthe mobile device; (ii) estimation of an altitude of the mobile devicebased on measurements of pressure form the mobile device andtransmitter(s), temperature measurement(s) from the transmitter(s) oranother source, and any other information needed for the computation);(iii) processing of received signals to determine position information(e.g., times of arrival or travel time of the signals, pseudorangesbetween the mobile device and transmitters, transmitter atmosphericconditions, transmitter and/or locations or other transmitterinformation); (iv) use of position information to compute an estimatedposition of the mobile device; (v) determination of movement based onmeasurements from inertial sensors of the mobile device; (vi) GNSSsignal processing; or (vii) other processing as required by operationsdescribed in this disclosure.

By way of example FIG. 8, the server may include: a mobile deviceinterface 21 for exchanging information with a mobile device (e.g., anantenna, a network interface, or other); one or more processor(s) 32;memory/data source 33 for providing storage and retrieval of informationand/or program instructions; a transmitter interface 34 for exchanginginformation with a transmitter (e.g., an antenna, a network interface,or other); and any other components known to one of ordinary skill inthe art. The memory/data source 33 may include memory storing softwaremodules with executable instructions, and the processor(s) 32 mayperform different actions by executing instructions from the modules,including: (i) performance of part or all of the methods as describedherein or otherwise understood by one of ordinary skill in the art asbeing performable at the server; (ii) estimation of an altitude of themobile device; (iii) computation of an estimated position of the mobiledevice; or (iv) other processing as required by operations described inthis disclosure. Steps performed by servers as described herein may alsobe performed on other machines that are remote from a mobile device,including computers of enterprises or any other suitable machine.

Systems and methods disclosed herein may operate within a network ofterrestrial transmitters or satellites. The transmitters may be locatedat different altitudes or depths that are inside or outside variousnatural or manmade structures (e.g. buildings). Positioning signals maybe sent to the mobile device from the transmitters and/or satellitesusing known wireless or wired transmission technologies. Thetransmitters may transmit the signals using one or more commonmultiplexing parameters—e.g. time slot, pseudorandom sequence, frequencyoffset, or other. The mobile device may take different forms, includinga mobile phone, a tablet, a laptop, a tracking tag, a receiver, oranother suitable device that can receive the positioning signals.Certain aspects disclosed herein relate to estimating the positions ofmobile devices—e.g., where the position is represented in terms of:latitude, longitude, and/or altitude coordinates; x, y, and/or zcoordinates; angular coordinates; or other representations. Varioustechniques to estimate the position of a mobile device can be used,including trilateration, which is the process of using geometry toestimate the position of a mobile device using distances traveled bydifferent “positioning” (or “ranging”) signals that are received by themobile device from different beacons (e.g., terrestrial transmittersand/or satellites). If position information like the transmission timeand reception time of a positioning signal from a beacon are known, thenthe difference between those times multiplied by speed of light wouldprovide an estimate of the distance traveled by that positioning signalfrom that beacon to the mobile device. Different estimated distancescorresponding to different positioning signals from different beaconscan be used along with position information like the locations of thosebeacons to estimate the position of the mobile device. Positioningsystems and methods that estimate a position of a mobile device (interms of latitude, longitude and/or altitude) based on positioningsignals from beacons (e.g., transmitters, and/or satellites) and/oratmospheric measurements are described in co-assigned U.S. Pat. No.8,130,141, issued Mar. 6, 2012, and U.S. Pat. Pub. No. 2012/0182180,published Jul. 19, 2012. It is noted that the term “positioning system”may refer to satellite systems (e.g., Global Navigation SatelliteSystems (GNSS) like GPS, GLONASS, Galileo, and Compass/Beidou),terrestrial transmitter systems, and hybrid satellite/terrestrialsystems.

This application relates to the following related application(s): U.S.Pat. Appl. No. 62/679,937, filed 3 Jun. 2018, entitled SYSTEMS ANDMETHODS FOR DETERMINING CALIBRATION VALUES FOR ATMOSPHERIC SENSORS THATPROVIDE MEASURED PRESSURES USED FOR ESTIMATING ALTITUDES OF MOBILEDEVICES. The content of each of the related application(s) is herebyincorporated by reference herein in its entirety.

1. A method for determining calibration values for atmospheric sensorsthat provide measured pressures used for estimating altitudes of mobiledevices, the method comprising: for different periods of time,determining an uncalibrated reference-level pressure estimate associatedwith an unstable pressure sensor and a calibrated reference-levelpressure estimate associated with a stable pressure sensor; determiningif the uncalibrated reference-level pressure estimates include anyuncalibrated reference-level pressure estimate that should not be usedwhen calibrating the unstable pressure sensor; if the uncalibratedreference-level pressure estimates do not include any uncalibratedreference-level pressure estimate that should not be used whencalibrating the unstable pressure sensor, determining a calibrationvalue for the unstable pressure sensor using all of the uncalibratedreference-level pressure estimates; and if the uncalibratedreference-level pressure estimates include an uncalibratedreference-level pressure estimate that should not be used whencalibrating the unstable pressure sensor: identifying at least oneuncalibrated reference-level pressure estimate that should not be usedwhen calibrating the unstable pressure sensor; and determining acalibration value for the unstable pressure sensor using all of theuncalibrated reference-level pressure estimates except any identifieduncalibrated reference-level pressure estimate that should not be usedwhen calibrating the unstable pressure sensor.
 2. The method of claim 1,wherein determining if the uncalibrated reference-level pressureestimates include any uncalibrated reference-level pressure estimatethat should not be used when calibrating the unstable pressure sensorcomprises: determining an initial calibration value using theuncalibrated reference-level pressure estimates and the calibratedreference-level pressure estimate; for each of the periods of time,determining an adjusted reference-level pressure estimate for the periodof time by adjusting the uncalibrated reference-level pressure estimatefor the period of time by the initial calibration value; for each of theperiods of time, determining a difference between (i) the adjustedreference-level pressure estimate for the period of time and (ii) thecalibrated reference-level pressure estimate for the period of time;determining if a threshold amount of the determined differences are lessthan a threshold pressure difference; if each determined difference inthe threshold amount of the determined differences is less than thethreshold pressure difference, determining that the uncalibratedreference-level pressure estimates do not include any uncalibratedreference-level pressure estimate that should not be used whencalibrating the unstable pressure sensor; and if each determineddifference in the threshold amount of the determined differences is notless than the threshold pressure difference, determining that theuncalibrated reference-level pressure estimates include an uncalibratedreference-level pressure estimate that should not be used whencalibrating the unstable pressure sensor.
 3. The method of claim 1,wherein determining if the uncalibrated reference-level pressureestimates include any uncalibrated reference-level pressure estimatethat should not be used when calibrating the unstable pressure sensorcomprises: for each of the periods of time, determining a differencebetween (i) the uncalibrated reference-level pressure estimate for theperiod of time and (ii) the calibrated reference-level pressure estimatefor the period of time); determining a mean or median of thedifferences; determining if a threshold amount of the determineddifferences are within a threshold pressure difference from the mean ormedian of the differences; if each determined difference in thethreshold amount of the determined differences is within the thresholdpressure difference from the mean or median of the differences,determining that the uncalibrated reference-level pressure estimates donot include any uncalibrated reference-level pressure estimate thatshould not be used when calibrating the unstable pressure sensor; and ifeach determined difference in the threshold amount of the determineddifferences is not within the threshold pressure difference from themean or median of the differences, determining that the uncalibratedreference-level pressure estimates include an uncalibratedreference-level pressure estimate that should not be used whencalibrating the unstable pressure sensor.
 4. The method of claim 1,wherein identifying at least one uncalibrated reference-level pressureestimate that should not be used when calibrating the unstable pressuresensor comprises: for each of the periods of time, determining adifference between (a) the uncalibrated reference-level pressureestimate of the time period and (b) the calibrated reference-levelpressure estimate for the period of time; for each of the periods oftime, determining if the determined difference for the period of timemeets a threshold condition; if the determined difference for the periodof time meets the threshold condition, identifying the uncalibratedreference-level pressure estimate as one of the uncalibratedreference-level pressure estimates that should not be used whencalibrating the unstable pressure sensor; and if the determineddifference for the period of time does not meet the threshold condition,not identifying the uncalibrated reference-level pressure estimate asone of the uncalibrated reference-level pressure estimates that shouldnot be used when calibrating the unstable pressure sensor.
 5. The methodof claim 1, wherein identifying at least one uncalibratedreference-level pressure estimate that should not be used whencalibrating the unstable pressure sensor comprises: determining aninitial calibration value using the uncalibrated reference-levelpressure estimates and the calibrated reference-level pressureestimates; for each of the periods of time, determining an adjustedreference-level pressure estimate for the period of time by adjustingthe uncalibrated reference-level pressure estimate for the period oftime by the initial calibration value; for each of the periods of time,determining a difference between (a) the adjusted reference-levelpressure estimate of the time period and (b) the calibratedreference-level pressure estimate for the period of time; for each ofthe periods of time, determining if the determined difference for theperiod of time meets a threshold condition; if the determined differencefor the period of time meets the threshold condition, identifying theuncalibrated reference-level pressure estimate as one of theuncalibrated reference-level pressure estimates that should not be usedwhen calibrating the unstable pressure sensor; and if the determineddifference for the period of time does not meet the threshold condition,not identifying the uncalibrated reference-level pressure estimate asone of the uncalibrated reference-level pressure estimates that shouldnot be used when calibrating the unstable pressure sensor.
 6. One ormore non-transitory machine-readable media embodying programinstructions that, when executed by one or more machines, cause the oneor more machines to implement method for determining calibration valuesfor atmospheric sensors that provide measured pressures used forestimating altitudes of mobile devices, the method comprising: fordifferent periods of time, determining an uncalibrated reference-levelpressure estimate associated with an unstable pressure sensor and acalibrated reference-level pressure estimate associated with a stablepressure sensor; determining if the uncalibrated reference-levelpressure estimates include any uncalibrated reference-level pressureestimate that should not be used when calibrating the unstable pressuresensor; if the uncalibrated reference-level pressure estimates do notinclude any uncalibrated reference-level pressure estimate that shouldnot be used when calibrating the unstable pressure sensor, determining acalibration value for the unstable pressure sensor using all of theuncalibrated reference-level pressure estimates; and if the uncalibratedreference-level pressure estimates include an uncalibratedreference-level pressure estimate that should not be used whencalibrating the unstable pressure sensor: identifying at least oneuncalibrated reference-level pressure estimate that should not be usedwhen calibrating the unstable pressure sensor; and determining acalibration value for the unstable pressure sensor using all of theuncalibrated reference-level pressure estimates except any identifieduncalibrated reference-level pressure estimate that should not be usedwhen calibrating the unstable pressure sensor.
 7. The one or morenon-transitory machine-readable media claim 6, wherein determining ifthe uncalibrated reference-level pressure estimates include anyuncalibrated reference-level pressure estimate that should not be usedwhen calibrating the unstable pressure sensor comprises: determining aninitial calibration value using the uncalibrated reference-levelpressure estimates and the calibrated reference-level pressure estimate;for each of the periods of time, determining an adjusted reference-levelpressure estimate for the period of time by adjusting the uncalibratedreference-level pressure estimate for the period of time by the initialcalibration value; for each of the periods of time, determining adifference between (i) the adjusted reference-level pressure estimatefor the period of time and (ii) the calibrated reference-level pressureestimate for the period of time; determining if a threshold amount ofthe determined differences are less than a threshold pressuredifference; if each determined difference in the threshold amount of thedetermined differences is less than the threshold pressure difference,determining that the uncalibrated reference-level pressure estimates donot include any uncalibrated reference-level pressure estimate thatshould not be used when calibrating the unstable pressure sensor; and ifeach determined difference in the threshold amount of the determineddifferences is not less than the threshold pressure difference,determining that the uncalibrated reference-level pressure estimatesinclude an uncalibrated reference-level pressure estimate that shouldnot be used when calibrating the unstable pressure sensor.
 8. The one ormore non-transitory machine-readable media claim 6, wherein determiningif the uncalibrated reference-level pressure estimates include anyuncalibrated reference-level pressure estimate that should not be usedwhen calibrating the unstable pressure sensor comprises: for each of theperiods of time, determining a difference between (i) the uncalibratedreference-level pressure estimate for the period of time and (ii) thecalibrated reference-level pressure estimate for the period of time);determining a mean or median of the differences; determining if athreshold amount of the determined differences are within a thresholdpressure difference from the mean or median of the differences; if eachdetermined difference in the threshold amount of the determineddifferences is within the threshold pressure difference from the mean ormedian of the differences, determining that the uncalibratedreference-level pressure estimates do not include any uncalibratedreference-level pressure estimate that should not be used whencalibrating the unstable pressure sensor; and if each determineddifference in the threshold amount of the determined differences is notwithin the threshold pressure difference from the mean or median of thedifferences, determining that the uncalibrated reference-level pressureestimates include an uncalibrated reference-level pressure estimate thatshould not be used when calibrating the unstable pressure sensor.
 9. Theone or more non-transitory machine-readable media claim 6, whereinidentifying at least one uncalibrated reference-level pressure estimatethat should not be used when calibrating the unstable pressure sensorcomprises: for each of the periods of time, determining a differencebetween (a) the uncalibrated reference-level pressure estimate of thetime period and (b) the calibrated reference-level pressure estimate forthe period of time; for each of the periods of time, determining if thedetermined difference for the period of time meets a thresholdcondition; if the determined difference for the period of time meets thethreshold condition, identifying the uncalibrated reference-levelpressure estimate as one of the uncalibrated reference-level pressureestimates that should not be used when calibrating the unstable pressuresensor; and if the determined difference for the period of time does notmeet the threshold condition, not identifying the uncalibratedreference-level pressure estimate as one of the uncalibratedreference-level pressure estimates that should not be used whencalibrating the unstable pressure sensor.
 10. The one or morenon-transitory machine-readable media claim 6, wherein identifying atleast one uncalibrated reference-level pressure estimate that should notbe used when calibrating the unstable pressure sensor comprises:determining an initial calibration value using the uncalibratedreference-level pressure estimates and the calibrated reference-levelpressure estimates; for each of the periods of time, determining anadjusted reference-level pressure estimate for the period of time byadjusting the uncalibrated reference-level pressure estimate for theperiod of time by the initial calibration value; for each of the periodsof time, determining a difference between (a) the adjustedreference-level pressure estimate of the time period and (b) thecalibrated reference-level pressure estimate for the period of time; foreach of the periods of time, determining if the determined differencefor the period of time meets a threshold condition; if the determineddifference for the period of time meets the threshold condition,identifying the uncalibrated reference-level pressure estimate as one ofthe uncalibrated reference-level pressure estimates that should not beused when calibrating the unstable pressure sensor; and if thedetermined difference for the period of time does not meet the thresholdcondition, not identifying the uncalibrated reference-level pressureestimate as one of the uncalibrated reference-level pressure estimatesthat should not be used when calibrating the unstable pressure sensor.11. A system for determining calibration values for atmospheric sensorsthat provide measured pressures used for estimating altitudes of mobiledevice, the system comprising one or more machines configured to performthe method of claim 1.